Petrol driven internal combustion engines are normally provided with a butterfly valve which regulates the flow of air supplied to the cylinders. Typically, the butterfly valve comprises a valve body housing a valve seat engaged by a butterfly disc which is keyed on a shaft in order to rotate between an open position and a closed position under the action of an electric motor connected to this shaft by means of a gear transmission. The shaft bearing the butterfly valve is associated with a position sensor which is adapted to detect the angular position of the shaft and therefore of the butterfly valve in order to enable a control unit to control, by feedback, the electric motor which determines the position of the butterfly valve.
The position sensor comprises a rotor which is mounted on the shaft and a stator which, in operation, faces the rotor and is adapted to detect the angular position of this rotor; the position sensor may of the “contact” type (typically formed by a potentiometer), i.e. there is a mechanical connection between the rotor and the stator, or of the “contact-free” type, i.e. there is no mechanical connection between the rotor and the stator of the position sensor. An embodiment of an inductive position sensor of a “contact-free” type normally used to detect the angular position of the shaft of an electronically controlled butterfly valve is disclosed in U.S. Pat. No. 6,236,199-B1.
The use of inductive position sensors of the “contact-free” type to detect the angular position of the shaft of an electronically controlled butterfly valve has recently been preferred as this type of sensor should ensure greater precision, higher speeds of reading and, in particular, a reading precision which is constant over time in comparison with the sensors of the “contact” type based on potentiometers widely used in the past. In some cases it has been observed, however, that once the production of the butterfly valve is complete, the performance of the inductive position sensor is substantially below its nominal performance, even though the position sensor is free from evident constructional defects.